At present, cell handover and reselection performed by a terminal is an important technique in a cellular mobile communication system. Specifically, when a user equipment (UE) leaves a cell and enters another one, a signal received from the original cell by the UE will become weaker, while a signal received from the cell that the UE is entering will get stronger. In order to maintain the quality of the signal received by the UE, the UE will transfer from the original base station to the new base station with a stronger signal.
With regard to a terminal in a connected state, this transfer between base stations is called handover. With regard to a terminal in an idle state, the change of a cell where the terminal camps is implemented via a cell reselection process, so it is necessary for the terminal to detect a target cell and perform cell reselection among the cells that meet requirements.
In a 3rd Generation (3G) mobile communication system, a terminal measures the quality of signals in different cells and compares the quality of the measured signals with the quality threshold of the signal in the serving cell. If the quality of a measured signal continuously meets the reselection standard within a specific time, the terminal will perform cell reselection and transfer to the cell with such signal quality.
Specifically, when the terminal performs cell reselection, it is required to measure the signal level or signal quality of its serving cell and neighbouring cells, then map the measured value of this serving cell into a cell reselection assessment value Rs, and map the measured value of a neighbouring cell into Rn. The Rs is compared with each Rn, and the cell ranked first is the cell with the best signal quality. When the best cell is not the cell where the terminal currently camps, the terminal will perform reselection and transfer to the best cell. Wherein the method of calculating Rs and Rn is determined by standard specifications. Parameters relating to calculation are distributed to the terminal via a broadcast channel of the serving cell.
An evolved universal terrestrial radio access network (E-UTRAN) system in a 3G mobile communication long term evolution (LTE) system continues to use the foregoing cell reselection method to perform intra-frequency cell reselection. Specifically, the terminal is required to calculate Rs=Qmeas,s+Qhysts, Rn=Qmeas,n−Qoffset, wherein s stands for a serving cell, n stands for a neighbouring cell, Qmeas,s stands for a measured level of a serving cell, Qmeas,n stands for a measured level of a neighbouring cell, Qhysts stands for the hysteresis of a serving cell, and Qoffset stands for a level offset of a neighbouring cell. After the calculation is completed, the terminal will rank all R values. The one ranked first is the best cell, the terminal will select the best cell when performing reselection. However, considering that the difficulty of reselection among neighbouring cells lies on not only the radio quality of these cells but also on the value of Qoffset, the foregoing reselection method through ranking R values is also called an offset-based cell reselection method.
As it is necessary to measure all intra-frequency and inter-frequency neighbouring cells when the offset-based cell reselection method is adopted, the terminal will consume more electric energy. As inter-frequency measurement requires the terminal to switch the resonant frequency of a receiver, whereas intra-frequency measurement does not need so, the LTE groups all neighbouring cells by frequency. The terminal firstly ranks the frequencies based on their absolute priority and finds out the frequency with top priority, and then selects a cell on this frequency by the offset-based cell reselection method. This reselection method is called a priority-based reselection method, and the selected cell should meet the following conditions: SnonServingCell,x of a cell on a measured frequency is greater than a threshold value Threshx,high within TreselectionRAT, wherein x stands for the measured frequency, nonServingCell stands for the measured cell, SnonServingCell,x stands for the measured level of the measured cell, TreselectionRAT stands for the measured time of cell reselection, and Threshx,high stands for the threshold of reselection to a frequency with higher priority. A cell may be chosen as a reselected cell only when it meets the foregoing conditions. Moreover, the following should be noted when a cell is selected as a reselected cell:
1. if a cell on a frequency with higher priority meets the foregoing conditions and the terminal has camped in a serving cell for more than one second, then the terminal will perform reselection and transfer to this cell;
2. if no reselectable cell exist on the serving frequency or on the frequencies having the same priority with the serving frequency, and no cell on the frequencies with higher priority meets the foregoing conditions, moreover, the measured level SServingCell of the serving cell is lower than a threshold Threshserving,low, when SnonServingCell,x of a cell on a measured frequency with lower priority is greater than a threshold Threshx,low within TreselectionRAT, then the terminal will perform reselection and transfer to this cell, wherein x stands for the measured frequency with lower priority, SnonServingCell,x stands for the measured level of the measured cell, TreselectionRAT stands for the measured time of cell reselection, Threshserving,low stands for the low-level threshold of the serving cell, and Threshx,low stands for the threshold of reselection to a frequency with lower priority;
3. the offset-based cell reselection method is adopted for reselection among intra-frequency cells and cells on the frequencies with the same priority;
4. when multiple cells on a frequency meet the foregoing conditions, the terminal will select the best cell as the reselected target cell.
It can be seen from the above description that in related techniques, the priority-based reselection method only defines that a terminal can select the best cell by the offset-based cell reselection method (R value-based sequencing method) on condition that this reselection is performed among cells on the same frequency. However, for this case that several cells on each of frequencies with the same priority meet the reselection conditions of the priority-based reselection method, no effective solution has been provided on how to perform cell reselection among these cells.